1. Field of the Invention
The present invention provides a phase shifting mask (PSM), particularly an alternating phase shifting mask (Alt-PSM) that can rectify spherical aberration induced by semi-dense lines and that can be used in a double exposure lithographic process with a light source of 248 nm. Besides, this invention can expand process window.
2. Description of the Prior Art
In the circuit making processes, lithographic process has not only been mandatory technique but also played an important role in limiting feature size. Only by lithographic process, a wafer producer can precisely and clearly transfer a circuit pattern onto a substrate. In a lithographic process, a designed pattern, such as a circuit pattern or a doping pattern, is created on one or several photo masks, then the pattern on the mask is transferred by light exposure, with a stepper and scanner, onto a substrate. Recently, the most mature lithographic technique is optical lithographic technique, of which the light sources include KrF laser (248 nm), ArF laser (193 nm) and F2 laser (157 nm) . . . etc., among which KrF laser""s light exposure technique is the most developed. The other two light exposure techniques and non-optical lithographic techniques, such as extreme UV and e-beam, are still underdeveloped.
According to the International Technology Roadmap for Semiconductor (ITRS) published by Semiconductor Industry Association (SIF), the smallest size of a semiconductor device should be reduced to less than 0.13 nm (about 130 nm) by the year of 2002. The light exposure technique with an ArF laser, with wavelength of 193 nm, can meet this requirement, but the cost is too high. Therefore, in order to achieve the same lithography performance as ArF laser and to prolong the life of a 248 nm lithographic equipment, a new proposal combining the resolution enhancement technique (RET) and KrF laser""s light exposure technique, is being developed.
Recently, many RET techniques, which can reduce devices"" sizes to the smallest with a light source of certain wavelength, employ a double exposure lithography technique with an alternating phase shifting mask (Alt-PSM). Beside an alternating phase shifting mask, the double exposure lithography technique employs a trim mask to complete pattern transferring. This technique was first developed by Wang, et al. from Numerical Technologies, Inc., (NTI), and was presented in detail in U.S. Pat. No. 5,858,580. However, this optical proximity correction (OPC) technique with Alt-PSM still has some problems, including transmission imbalance which occurred in phase shifted and non-phase shifted regions and other flaws caused by Alt-PSM.
Besides, with the increase of devices"" packing density, pitches-particularly the ones between gate electrodes are reduced, resulting in some dense lines, semi dense lines or isolated lines on the photo mask. Unfortunately, conventional Alt-PSM can""t avoid spherical aberration caused by these semi-dense line patterns, narrowing the process window created by the dense lines and the semi-dense lines formed on the resist, and hampering the quality of the lithographic process.
Please refer to FIG. 1; FIG. 1 is an enlarged partial view of an Alt-PSM according to the prior art. In order to explain the faults of the prior technique, FIG. 1 only shows the region 10 related to the present invention. As shown in here, the region 10 comprises a dense-line region 12 and a semi-dense-line region 14, wherein the dense-line region 12 comprises two closely arrayed dense lines 4a and b formed on the quartz mask. The dense lines 4a and b and the semi-dense line 5 are made of opaque material, such as chromium (CR). The pitch 20 is the sum of line width (L) and the line space (S). Usually, the semi-dense line 5 has a pitch of less than 400 nm from the dense lines 4a and b on its one side and a pitch larger than 400 nm on the other side. Moreover, between the dense line 4a and the dense line b is a phase shifting region 1 of 180 degree, and between the dense line 4a, and the semi-dense line 5 a phase shifting region 2 of zero degree, and the other side of the semi-dense line 5 is a phase shifting region 3 of 180 degree.
Please refer to FIG. 2; FIG. 2 is a plot of center focus vs. pitch from an Alt-PSM according to the prior art, wherein the Y-axis is center focus in um, the X-axis is pitch in nm. As shown in the plot, when a pitch on an Alt-PSM exceeds 500 nm, namely a pattern comprising semi-dense lines, the center focus would shift; as a pith is between 300 nm and 400 nm, the center focus is between 0.45 and 0.4 um; as a pitch increases to between 650 nm and 700 nm, the center focus shifts to around 0.7 um and results in insufficient width of process window created by the dense lines (pitches less than 400 nm) and the semi-dense lines (pitches more than 400 nm). As shown in FIG. 3, in which the Y-axis is focus, the region 25 is the domain of the process window created by the dense-lines; the region 26 is the domain of the process window created by the semi-dense lines. Shifting of the center focus of the semi-dense line pattern results in a size-reduction of the region 27, which is the overlapping area of the region 25 and the region 26, and which represents the size of the process window in the light exposing process.
The main purpose of the present invention is to provide an Alt-PSM, which can correct the spherical aberration caused by semi-dense lines and can increase the size of process window, and which is to be employed in a double exposure lithographic process with a light source of 248 nm.
The phase shifting mask in the preferred embodiment of the present invention comprises: 1) a quartz substrate; 2) at least one semi-dense line formed on the substrate, wherein the semi-dense line is adjacent to a clear region with a width more than 2 nm on one side and to a dense line pattern with a narrow pitch on the other side; 3) a first phase shifting region, which is located between the dense line pa tern and the semi-dense line, adjacent to the semi-dense line; and 4) a second phase shifting region with a predetermined width, adjacent to the semi-dense line opposite to the first phase shifting region, wherein the phase difference between the first phase shifting region and the second phase shifting region is 180 degree.
The Alt-PSM in another preferred embodiment comprises: 1) a quartz substrate; 2) a dense line pattern formed on the quartz substrate; 3) at least one semi-dense line pattern adjacent to the dense-line pattern; 4) a first phase shifting region with a first predetermined width, located between the dense-line pattern and the semi-dense line pattern and adjacent to the dense-line pattern; 5) a second shifting region with a second predetermined width, which is unconnected to the first shifting region, and is located between the dense line pattern and the semi-dense line pattern and adjacent to the later; and 6) a third phase shifting region with the same width as the second phase shifting region, located on the side opposite to the second phase shifting region and adjacent to the semi-dense line pattern; wherein the phase difference between the first and second phase shifting regions is zero degree, and the one between the second and third phase shifting regions is 180 degree.